Thanks for bringing up the topic. I too have been considering creating a GPL store before. From my research and experience, it has been a mixed bag.
On the positive side, there is a huge market for GPL products. I know that many WordPress users are always on the lookout for affordable plugins and theme options. If you have an extensive collection of GPL items, that will be a solid starting point for you.
However, the competition is fierce, as others have acknowledged. You are not just competing with other GPL stores, but also with official plugin/theme developers!
SEO can work for you, but it is quite challenging. You will need to focus on creating valuable content around WordPress to stand out. Just listing out the GPL products may not cut it.
You will also need to look at the technical side too. You need to stay on top of your game through updates, potential security concerns, and compatibility issues. You will not just do the role of distributing files, you will also assume the supporting role!
If you finally decide to go for it, consider starting small. Maybe you can focus on building your expertise by focusing on a specific niche within WordPress, like portfolio themes or e-commerce plugins. This will help you stand out in the crowded marketplace..